Cosmic cartographers find 'great wall.'

Cosmic Cartographers Find 'Great Wall'

Two astrophysicists mapping the locations of galaxies have found the largest structure known in the universe -- a thin sheet of galaxies half a billion light-years long. Dubbed the "Great Wall," it lies 200 to 300 million light-years from Earth and challenges the prevailing view of how matter became distributed in the universe, they say.

Astronomers have long assumed that on a very large scale, any place in the universe should contain the same amount of matter as any other location. Just as a choppy ocean looks smooth from a space shuttle, the universe as a whole should appear uniform, despite occasional small structures like galaxies.

The astronomical survey, conducted by Margaret J. Geller and John P. Huchra of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., looks at larger slices of the universe than any previous survey. Even at this scale, galaxies group together in surprisingly coherent arrangements, they report in the Nov. 17 SCIENCE.

Guided by a two-dimensional map made in the 1960s, Geller and Huchra have mapped more than 11,000 galaxies in a wedge of the northern celestial hemisphere, measuring the redshift of each galaxy brighter than magnitude 20.5 to pinpoint its location in three dimensions. The redshift indirectly indicates an object's distance from Earth. Their ongoing survey now covers 1/100,000 of the volume of the visible universe -- comparable to a world atlas describing only Rhode Island.

Stretching dramatically across the map is the Great Wall, a galaxy cluster 500 million light-years long, 200 million light-years wide and 15 million light-years thick. The vast bubbles it outlines -- nearly empty stretches of space 150 million light-years in diameter -- are equally important. "It may make more physical sense to regard the individual voids as the fundamental structures" of the universe, Geller and Huchra write. These voids threaten one theory describing the nature of dark matter -- the hypothetical material that caulks the gap between the amount of material astrophysicists detect in the universe and the much greater quantity they know must exist. Without dark matter to make up the difference, gravity couldn't hold stars and galaxies together.

Most astrophysicists believe dark matter is slow-moving, or "cold." In this standard model, galaxies form at denser regions of dark matter, as foam gathers at the tip of a wave, Geller and Huchra say. But the vast size of the newly mapped voids doesn't match the calculated distribution of cold dark matter, they report. Thus, the theory is missing "something profound," Geller says.

Not so, responds theoretical astrophysicist James E. Gunn of Princeton (N.J.) University. "I think [the new finding] actually poses no problem at all," he says. Last summer, Gunn and two graduate students modeled the growth of a universe containing cold dark matter. Their computer models simulated large voids and "remove any doubt" that these structures are compatible with the theory of cold dark matter, Gunn says.

Since Geller and Huchra reported their first results nearly four years ago (SN: 1/18/86, p. 38), many other astrophysicists have started their own mapping projects, some scrutinizing different regions of space and others looking at already-mapped regions in more detail, Geller says. But until surveyors map a region vast enough to show conclusively whether matter is evenly distributed, she says, they cannot accurately describe the structure of the universe.